Despite the advent and promise of recent technologies, including combinatorial chemistry, high-throughput screening, genomics, and proteomics, the number of new drugs and vaccines reaching the market has not increased. In fact, the attrition rate within drug discovery programs still exceeds 90%.
The introduction of these new (and expensive) technologies has not reduced the lost opportunity costs associated with immunotherapy development; rather, these costs have increased. Indeed, it is now estimated that almost $1 billion is required to bring a new drug to the market.
The development and biological testing of human vaccines has traditionally relied on small animal models (e.g., mouse and rabbit models) and then non-human primate models. However, such small animal models are expensive and non-human primate models are both expensive and precious. Furthermore, there are many issues regarding the value of such animal studies in predicting outcomes in human studies.
A major problem remains the translation from test systems to human immunology. Successful transfer between traditional testing systems and human biology requires an intricate understanding of disease pathogenesis and immunological responses at all levels. Given worldwide health problems caused by known and emerging infectious agents and even potential biological warfare pathogens, it is time for a fresh approach to understanding disease pathogenesis, the development and rapid testing of vaccines, and insights gathered from such work.
The body's distributed immune system can be roughly divided into four distinct compartments: tissues and blood, mucosal tissues, body cavities, and skin. Because of ease of study, most is known about the tissue and blood compartment and its lymphoid tissues, the spleen and lymph nodes.
The mammalian immune system uses two general mechanisms to protect the body against environmental pathogens. The immune system recognizes and responds to structural differences between self and non-self proteins. Proteins that the immune system recognizes as non-self are referred to as antigens. Pathogens typically express large numbers of complex antigens. When a pathogen-derived molecule is encountered, the immune response becomes activated to enable protection against that pathogenic organism.
The first immune system mechanism is the non-specific (or innate) inflammatory response. The innate immune system appears to recognize specific molecules that are present on pathogens but not on the body itself.
The second immune system mechanism is the specific or acquired (or adaptive) immune response; acquired immunity is mediated by specialized immune cells called B and T lymphocytes (or simply B and T cells). Acquired immunity has specific memory for antigenic structures; repeated exposure to the same antigen increases the response, which increases the level of induced protection against that particular pathogen. Whereas innate responses are fundamentally the same for each injury or infection, acquired responses are custom-tailored to the pathogen in question. The acquired immune system evolves a specific immunoglobulin (antibody) response to many different molecules present in the pathogen, called antigens. In addition, a large repertoire of T cell receptors (TCR) is sampled for their ability to bind processed forms of the antigens bound to major histocompatibility complex (MHC, also known as human leukocyte antigen, HLA) class I and II proteins on the surface of antigen-presenting cells (APCs), such as dendritic cells (DCs).
B cells produce and mediate their functions through the actions of antibodies. B cell-dependent immune responses are referred to as “humoral immunity,” because antibodies are found in body fluids.
T cell-dependent immune responses are referred to as “cell-mediated immunity,” because effector activities are mediated directly by the local actions of effector T cells. The local actions of effector T cells are amplified through synergistic interactions between T cells and secondary effector cells, such as activated macrophages. The result is that the pathogen is killed and prevented from causing diseases.
The functional element of mammalian secondary lymphoid tissues, including lymph nodes, is the follicle, which develops a germinal center (GC) when stimulated by an antigen. The GC is an active area within a lymph node, where important interactions occur in the development of an effective humoral immune response. Upon antigen stimulation, follicles are replicated and an active human lymph node may have dozens of active follicles, with functioning GCs. Interactions between B cells, T cells, and FDCs take place in GCs.
Various studies of GCs in vivo indicate that the many important events occur there, including immunoglobulin (Ig) class switching, rapid B cell proliferation (GC dark zone), production of B memory cells, accumulation of select populations of antigen-specific T cells and B cells, hypermutation, selection of somatically mutated B cells with high affinity receptors, apoptosis of low affinity B cells, affinity maturation, induction of secondary antibody responses, and regulation of serum immunoglobulin G (IgG) with high affinity antibodies. Similarly, data from in vitro GC models indicate that FDCs are involved in stimulating B cell proliferation with mitogens and it can also be demonstrated with antigen (Ag), promoting production of antibodies including recall antibody responses, producing chemokines that attract B cells and certain populations of T cells, immunoglobulin class switching, somatic hypermutation, selection of high affinity B cells, promoting production of high affinity antibodies, and blocking apoptosis of B cells.
Similar to pathogens, vaccines function by initiating an innate immune response at the vaccination site and activating antigen-specific T and B cells that can give rise to long term memory cells in secondary lymphoid tissues. The precise interactions of the vaccine with cells at the vaccination site and with T and B cells of the lymphoid tissues are important to the ultimate success of the vaccine.
Almost all vaccines to infectious organisms were and continue to be developed through the classical approach of generating an attenuated or inactivated pathogen as the vaccine itself. This approach, however, fails to take advantage of the recent explosion in our mechanistic understanding of immunity. Rather, it remains an empirical approach that consists of making variants of the pathogen and testing them for efficacy in non-human animal models.
Advances in the design, creation and testing of more sophisticated vaccines have been stalled for several reasons. First, only a small number of vaccines can be tested in humans, because, understandably, there is little societal tolerance for harmful side effects in healthy people, especially children, exposed to experimental vaccines. With the exception of cancer vaccine trials, this greatly limits the innovation that can be allowed in the real world of human clinical trials. Second, it remains challenging to predict which epitopes are optimal for induction of immunodominant CD4 and CD8 T cell responses and neutralizing B cell (antibody) responses. Third, small animal testing, followed by primate trials, has been the mainstay of vaccine development; such approaches are limited by intrinsic differences between human and non-human species, and ethical and cost considerations that restrict the use of non-human primates. Consequently, there has been a slow translation of basic knowledge to the clinic, but equally important, a slow advance in the understanding of human immunity in vivo.